Chapter 7 Computer Control Fundamentals 131
Oxygen Sensor Location
All vehicles built after 1995 are equipped with at least
two oxygen sensors. One is mounted before the catalytic
converter and monitors O
2
levels in the exhaust gas as it
enters the converter. The other sensor, which is referred
to as a catalyst monitor, is mounted behind the catalytic
converter. This sensor monitors O
2
levels of the exhaust gas
as it leaves the converter. The second O
2
sensor also acts as
a backup should the first one fail.
The oxygen sensor location is designated by engine
bank (or side) and sensor position in relation to the engine,
Figure 7-21. For example, the sensor might be designated
as bank 2, sensor 1. This indicates it is located on the engine
side opposite of the number one cylinder and is the sensor
on that side nearest to the engine. The farthest sensor is
always the catalyst monitor. This is usually bank 2, sensor 2
or 3, depending on the number of oxygen sensors.
A variation of the O2 sensor is the NOX sensor. The
NOX sensor is made of zirconia like the O2 sensor, but is
configured in a slightly different way. The platnum elec-
trodes are molded into the element surface instead of being
inside of the ceramic element. The resulting electrical sig-
nal can be measured by the ECM as NOX concentration. In
Figure 7-21, NOX sensors are shown installed behind the
NOX storage-type catalytic converters.
Drive Train Sensors
Drive train sensors monitor conditions in the transmis-
sion/transaxle and other drive train components. Most drive
train sensors are driver operated. A driver-operated sensor
is any sensor that detects the operation of a device, system,
or component commanded by the driver. These sensors are
usually installed on the throttle linkage, brake pedal, shift
lever, and other drive train and chassis systems.
Vehicle Speed Sensors
Most vehicle speed sensors (VSS) are mounted in the
transmission/transaxle case at a point where the sensor can
monitor the speed of the output shaft, Figure 7-22. Most
vehicle speed sensors are permanent magnet generators.
Other speed sensors include photoelectric sensors mounted
in the speedometer housing. Unlike most sensors, speed
sensors produce alternating current.
Some vehicle speed sensors are mounted on the drive
wheels to allow the ECM to control wheel traction or adjust
engine settings for lowest emissions. The same wheel sen-
sors are often used as inputs to the anti-lock brake module.
The operation of most vehicle speed sensors is based on the
same principles as engine speed sensors.
Reed Switches
Reed switches are often used to measure speed by
determining rotation. A typical reed switch is shown in
Figure 7-23. The reed switch consists of two thin metal
blades, or reeds, that contact each other inside of a closed
chamber. The metal reeds can flex without breaking. A small
amount of current flowing in the reeds creates a magnetic
field, which attracts the reeds to each other. Any outside
magnetic field, however, will cause the reeds to repel each
other and move apart.
Figure 7-21. Oxygen sensor location in the exhaust system. Bank 1 is the side of the engine that has the number one cylinder.
Sensor 1 is the sensor closest to the number one cylinder. (Daimler)
Bank 1, Sensor 1
Bank 2, Sensor 1
Bank 2, Sensor 2
Bank 1, Sensor 2
Temperature sensor
NOX storage-type catalytic converters
NOX sensors
Catalytic
converter
Catalytic converter